CODEWITHSUNDEEP
c++c++11shared-ptr
Narek
Narek

Reputation: 39871

How std::enable_shared_from_this::shared_from_this works

I just cannot understand how std::enable_shared_from_this::shared_from_this returns a shared pointer that shared ownership with existing pointer. In other words you do this:

std::shared_ptr<Foo> getFoo() { return shared_from_this(); }

So when you call getFoo how does exactly it get what is the other shared_ptr to share the ownership with and not to create a separate shared_ptr that owns the same this.

I need to understand this to be able to understand how to create shared_ptr from some object that all increase the same ref count and not initialize separate shared_ptrs.

Upvotes: 28

Views: 8646

Answers (1)

Simple
Simple

Reputation: 14390

enable_shared_from_this<T> has a weak_ptr<T> data member. The shared_ptr<T> constructor can detect if T is derived from enable_shared_from_this<T>. If it is, the shared_ptr<T> constructor will assign *this (which is the shared_ptr<T>) to the weak_ptr data member in enable_shared_from_this<T>. shared_from_this() can then create a shared_ptr<T> from the weak_ptr<T>.

Example of a possible implementation:

template<class D>
class enable_shared_from_this {
protected:
    constexpr enable_shared_from_this() { }
    enable_shared_from_this(enable_shared_from_this const&) { }
    enable_shared_from_this& operator=(enable_shared_from_this const&) {
        return *this;
    }

public:
    shared_ptr<T> shared_from_this() { return self_.lock(); }
    shared_ptr<T const> shared_from_this() const { return self_.lock(); }

private:
    weak_ptr<D> self_;

    friend shared_ptr<D>;
};

template<typename T>
shared_ptr<T>::shared_ptr(T* ptr) {
    // ...
    // Code that creates control block goes here.
    // ...

    // NOTE: This if check is pseudo-code. Won't compile. There's a few
    // issues not being taken in to account that would make this example
    // rather noisy.
    if (is_base_of<enable_shared_from_this<T>, T>::value) {
        enable_shared_from_this<T>& base = *ptr;
        base.self_ = *this;
    }
}

Upvotes: 46

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